The present invention generally relates to a starter, and particularly to an assembling structure of a yoke, a brush device and a rear bracket.
First, a related art will be described below with reference to FIG. 10 which is a sectional view showing a conventional starter. Hereinafter, the right side and left side on the paper of the drawings are referred to as "front side" and "rear side", respectively. In FIG. 10, an armature 21 is enclosed in a cylindrical yoke 22. A cylindrical portion of a metal cup-like rear bracket 23 is attached to the rear end portion of the yoke 22 through spigot joint. A base 24 is fixed to the rear end inner surface of the rear bracket 23. Resin brush holding frames 25 are supported in the inside of the base 24. The armature 21 is constituted by an armature rotation shaft 26, a commutator 27 having segments formed on its outer circumference, and an armature coil 28 connected to the segments of the commutator 27. A bearing 29 which supports the armature rotation shaft 26 is fitted in the inner surface of an annular projection portion 23a of the rear bracket 23. Plus- and minus-side brushes 30 are received in the brush holding frames 25 and located in position respectively. The brushes 30 are pressed against the commutator 27 by means of brush springs 31. Further, a front bracket 32 is disposed on the front end portion of the starter, and a magnet switch 33 is provided on the upper portion of the starter.
In the aforementioned configuration, because the inner diameter of the rear bracket 23 fitted to the yoke 22 is smaller than the outer diameter of the yoke 22 and a brush device including the brush holders and brushes is received in the rear bracket 23, the outermost circumferential portion of the brush device becomes smaller than the inner diameter of the rear bracket 23. Accordingly, the outermost circumferential portion of the brush device must be smaller than the outer diameter of the yoke 22.
The operation of the starter will be described below. A current flows into a segment of the commutator 27 via the plus-side brush 30, and flows into the armature coil 28 from the segment of the commutator 27. Then, the current flowing-out from another segment of the commutator 27 flows, via the minus-side the brushes 30, into the base 24, the rear bracket 23 and the yoke 22 in the order successively so as to be finally grounded via the yoke 22.
Because the conventional starter is configured as described above, the inner diameter of the rear bracket is limited by the outer diameter of the yoke which is fitted to the rear bracket. Further, because the diameter of the outermost circumferential portion of the brush device is limited by the inner diameter of the rear bracket, the brushes which are constituent members of the brush device for supplying a current to the armature are limited in their length. It is therefore impossible to make the brushes have sufficient length, and it is impossible to attain a necessary brush life.
In the conventional structure, as described above, the brush life of the starter is be short. If the brush life is required to be prolonged in this case, it may be thought of to use brushes containing a large amount of carbon and a small amount of copper to increase their electric resistance. Such brushes are, however, unsuitable for a starter which requires large electric power instantaneously.